When stimulation comes in from several sources and some processing is called for that logically requires information from all the sources, can the processing of all the information be carried out simultaneously, or must the organism process each source of stimulation in turn? The data from the visual processing literature suggest that processing of several aspects or dimensions of a single stimulus can go on simulataneously, but that most kinds of processing from several spatial positions involve serial processing of those positions. The proposed research combines two of the tasks of the visual perception literature, the dimensional processing task and the visual search task, in order to explore the limits of parallel independent processing. A simple example of the paradigm proposed is: the subject responds on the basis of whether either of two figures is a red square. This extension of the simpler dimensional processing task (using only one figure) is designed to explore the limits of parallel (simultaneous) processing. Furthermore, the paradigm can be used to test whether different spatial positions are processed independently. If they are, then when the criterial attributes (e.g., red and square) are in different spatial locations, the subject should have no trouble in determining that there is no red square present. The above condition of spatial independence can be used as a defining criterion of a perceptual unit.